High-efficiency multiwavelength beam expander employing dielectric-enhanced mirrors

ABSTRACT

A high-efficiency, multiwavelength beam-expander optical system that employs dielectric-enhanced mirrors is disclosed. Each mirror includes a reflective multilayer coating formed from alternating layers of HfO 2  and SiO 2  that define, in order from the substrate surface, at least first and second sections, wherein the HfO 2 /SiO 2  layer thicknesses are generally constant within a given section and get smaller section by section moving outward from the substrate surface. The first and second sections are respectively configured to optimally reflect different operating wavelengths so that the beam-expander optical system has an optical transmission of greater than 95% at the different operating wavelengths.

This application is a divisional and claims the benefit of priorityunder 35 U.S.C. § 120 of U.S. patent application Ser. No. 14/854,739,filed on Sep. 15, 2015, which claims the benefit of priority under 35U.S.C. § 119 of U.S. Provisional Application Ser. No. 62/051,354 filedon Sep. 17, 2014, the contents of which are relied upon and incorporatedherein by reference in their entirety.

FIELD

The present disclosure relates to a beam expander, and in particular toa high-efficiency multiwavelength beam expander that employsdielectric-enhanced mirrors.

The entire disclosure of any publication or patent document mentionedherein is incorporated by reference, including U.S. Pat. No. 7,683,450,entitled “Method for producing smooth dense optical films” and which isreferred to below as the '450 patent, and the article by Wang et al.,“HfO₂/SiO₂ enhanced diamond turned aluminum mirrors for IR laseroptics,” Proc. SPIE 8190, 8190005 (2011).

BACKGROUND

Beam expanders are used to enlarge the size of a beam of light. Beamexpanders are often used in laser applications where a laser beam from alight source starts out having a very small diameter but needs to beexpanded in at least one direction for use downstream. In many cases,the laser beam has a high intensity and needs to be expanded to avoiddamaging the downstream optical components.

Some beam expanders are catoptric optical systems, i.e., all of theoptical components are reflective. Reflective optical elements are oftenpreferred for beam expanders that operate at multiple wavelengthsbecause they do not suffer from chromatic aberration and therefore donot require color correction.

FIG. 1 is a plot of the reflectance R (%) versus wavelength A (nm) for adiamond-turned and optically polished aluminum mirror having a mirrorsubstrate made of aluminum alloy (Al 6061-T6). The mirror includes aconventional quarter-wavelength HfO₂/SiO₂ multilayer reflective coatingoptimized at A=1064 nm. The plot shows a broadband spectral reflectancefrom the ultraviolet (UV) to the near infrared (NIR), with reflectancesR of 92.3% at 355 nm, 92.0% at 532 nm and 95.0% at 1064 nm. The threewavelengths selected are the common output wavelengths for a high-powerNd:YAG laser. The reflectance R at 355 nm has a bandwidth of only 24 nm.In the visible spectral range, the average reflectance is similar tothat of bare aluminum.

For a beam expander that uses two of the aluminum mirrors having thereflectance properties of FIG. 1, the optical transmittance is 85.2% at355 nm, 84.6% at 532 nm and 90.3% at 1064 nm. There are three maindrawbacks associated with such a multiband laser beam expander, namely,a relatively low optical transmittance, a low laser-damage resistance,and susceptibility to surface degradation over time when exposed toextreme environments.

A beam expander for use with high-power lasers needs to have much higheroptical transmittance (e.g., >95%) at each of the designated (operating)wavelengths, and preferably is resistant to laser-damage resistance andrelatively insusceptible to surface degradation.

SUMMARY

An aspect of the disclosure is a beam-expander optical system thatincludes: a convex mirror having a first mirror substrate made of metaland having a convex substrate surface and a first reflective multilayercoating formed on the convex substrate surface; and a concave mirrorhaving a second mirror substrate made of the metal and having a concavesubstrate surface and a second reflective multilayer coating formed onthe concave substrate surface, with the convex and concave mirrors beingarranged in an off-axis, afocal configuration having greater than unitymagnification. The first and second reflective multilayer coatings eachincludes alternating layers of HfO₂ and SiO₂ that define at least firstand second sections S1 and S2. The first section S1 is closest to theconvex or concave substrate surface and is configured to optimallyreflect a first wavelength of light and substantially transmitmid-wavelength IR (MWIR) light. The second section S2 resides atop thefirst section and is configured to optimally reflect a second wavelengthof light that is shorter than the first wavelength of light. The secondsection S2 also substantially transmits the MWIR light. Thebeam-expander optical system has a high-efficiency transmittanceT_(BE)>95% for the first and second wavelengths of light.

Another aspect of the disclosure is the beam-expander optical system asdescribed above, wherein the first and second multilayer coatings eachincludes a third section S3 atop the second section S2. The thirdsection S3 is configured to optimally reflect a third wavelength oflight that is shorter than the second wavelength of light andsubstantially transmit the MWIR light. In an example, the firstwavelength of light is an infrared wavelength, the second wavelength oflight is a visible wavelength and the third wavelength of light is anultraviolet wavelength.

Another aspect of the disclosure is a high-efficiency beam-expanderoptical system for use at UV, VIS and IR operating wavelengths. Thebeam-expander optical system includes: a first mirror with a convexsubstrate surface formed on a first Al alloy substrate and a secondmirror with a concave substrate surface formed on a second Al alloysubstrate. The first and second mirrors are arranged in an off-axis,afocal configuration having greater than unity magnification. The convexand concave substrate surfaces each include a multilayer reflectivecoating formed from alternating layers of HfO₂ and SiO₂ havingrespective layer thicknesses τ_(H) and τ_(S). Each multilayer reflectivecoating includes: i) a first section immediately adjacent the convex orconcave reflective surface and configured to optimally reflect the IRoperating wavelength and substantially transmit a mid-wavelength IR(MWIR) wavelength; ii) a second section atop the first section andconfigured to optimally reflect the VIS operating wavelength andsubstantially transmit the IR and MWIR operating wavelengths; and iii) athird section atop the second section and configured to optimallyreflect the UV operating wavelength and substantially transmit the VIS,IR and MWIR wavelengths. The respective HfO₂ and SiO₂ layer thicknessesτ_(H) and τ_(S) are substantially constant within each section but thethicknesses in the second section are smaller on average than those inthe first section and the thicknesses in the third section are smalleron average than those in the second section. The beam-expander opticalsystem has a transmittance T_(BE)>95% at the UV light, the VIS light andthe IR operating wavelengths.

Another aspect of the disclosure is a method of forming ahigh-efficiency beam-expander optical system for use at UV, VIS and IRoperating wavelengths. The method includes: diamond-turning andpolishing first and second metal substrates to respectively form a firstmirror having a convex substrate surface and a second mirror having aconcave substrate surface; forming on each of the convex and concavesubstrate surfaces a reflective multilayer coating consisting ofalternating layers of HfO₂ and SiO₂ having respective layer thicknessesτ_(H) and τ_(S), including arranging the HfO₂ and SiO₂ layers in atleast three sections S1, S2 and S3 in order outward from the convex orthe concave surface, the three sections being respectively configured tooptimally reflect the IR, VIS and UV operating wavelengths; andarranging the first and second mirrors in an off-axis, afocalconfiguration having greater than unity magnification and an opticaltransmittance T_(BE)>95% at each of the UV, VIS and IR operatingwavelengths. In an example, the metal mirror substrates are made of analuminum alloy.

Additional features and advantages are set forth in the DetailedDescription that follows, and in part will be readily apparent to thoseskilled in the art from the description or recognized by practicing theembodiments as described in the written description and claims hereof,as well as the appended drawings. It is to be understood that both theforegoing general description and the following Detailed Description aremerely exemplary, and are intended to provide an overview or frameworkto understand the nature and character of the claims.

BRIEF DESCRIPTION OF τHE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding and are incorporated in and constitute a part of thisspecification. The drawings illustrate one or more embodiment(s) andtogether with the Detailed Description serve to explain the principlesand operation of the various embodiments. As such, the disclosure willbecome more fully understood from the following Detailed Description,taken in conjunction with the accompanying Figures, in which:

FIG. 1 is a plot of the reflectance R (%) versus wavelength λ (nm) for adiamond-turned and optically polished aluminum mirror having a mirrorsubstrate made of an aluminum alloy (Al 6061-T6) and including aconventional quarter-wavelength HfO₂/SiO₂ multilayer reflective coatingoptimized at λ=1064 nm, wherein the plot illustrates relatively lowreflectances at operating wavelengths λ of 355 nm, 532 nm and 1064 nm;

FIG. 2 is a schematic diagram of an example two-mirror, off-axis beamexpander according to the disclosure, shown along with a laser source,wherein each mirror includes the HfO₂/SiO₂ reflective multilayer coatingas disclosed herein that provides the beam expander with an opticaltransmittance T_(BE)>95% for ultraviolet, visible and infrared operatingwavelengths;

FIG. 3 is a close-up, cross-sectional view of an example mirror used inthe beam expander of FIG. 2 and shows a portion of the mirror substrateand the corresponding reflective multilayer coating formed thereon,illustrating how the coating layers have a thickness that generallydecreases with distance (z-direction) from the substrate surface;

FIG. 4 is a plot of the refractive index n (at 355 nm) versus thedistance z (nm), illustrating an example of how the HfO₂/SiO₂ layers ofthe reflective multilayer coating define different sections S1 throughS3 having different (average) layer thicknesses;

FIG. 5 is a plot of the reflectance R (%) versus wavelength λ (nm) foran example beam-expander mirror with a reflective multilayer coatinghaving a varying-thickness configuration such as shown in FIG. 4;

FIG. 6 is a cross-sectional diagram of an example beam-expander mirrorillustrating how the reflective multilayer coating includes threesections S1, S2 and S3, with the uppermost section S3 reflecting UVlight, the middle section S2 reflecting visible light, and the lowermostsection S1 reflecting NIR light, while the substrate surface reflectsMWIR light;

FIG. 7 is a plot of the reflectance R (%) versus wavelength λ (nm)illustrating an example reflectance spectrum for an examplebeam-expander mirror having a reflective multilayer stack configured forhigh-efficiency reflectance at operating wavelengths λ of 355 nm, 532 nmand 1064 nm, as well as at SWIR wavelengths;

FIG. 8 is similar to FIG. 7 and illustrates an example reflectancespectrum for an example beam-expander mirror having a reflectivemultilayer stack configured for high-efficiency reflectance at operatingwavelengths λ of 355 nm, 532 nm, 1064 nm, SWIR wavelengths and MWIRwavelengths;

FIG. 9 is a plot of the scattering loss SL (%) versus theroot-mean-square (RMS) substrate surface roughness MSR_(RMS)(nm) fordifferent operating wavelengths λ of 355 nm, 532 nm and 1064 nm,illustrating the impact of the substrate surface roughness on thescattering loss due to optical scattering of light at the differentoperating wavelengths;

FIG. 10 is a plot of the optical transmittance T_(BE)(%) of the beamexpander of FIG. 2 as a function of the RMS substrate surface roughnessMSR_(RMS)(nm) for different operating wavelengths λ of 355 nm, 532 nmand 1064 nm, illustrating the impact of the substrate surface roughnesson the beam expander optical transmittance and the need to maintain theRMS substrate surface roughness below a threshold value in order toachieve a high-efficiency optical transmittance of T_(BE)>95%; and

FIGS. 11A through 11C are schematic side views that illustrate anexample method of mitigating defects on the substrate surface using anintermediate SiO₂ layer that is processed using plasma-ion etching priorto the formation of the HfO₂/SiO₂ reflective multilayer coating asdisclosed herein.

DETAILED DESCRIPTION

Reference is now made in detail to various embodiments of thedisclosure, examples of which are illustrated in the accompanyingdrawings. Whenever possible, the same or like reference numbers andsymbols are used throughout the drawings to refer to the same or likeparts. The drawings are not necessarily to scale, and one skilled in theart will recognize where the drawings have been simplified to illustratethe key aspects of the disclosure.

The claims as set forth below are incorporated into and constitute apart of this Detailed Description.

Cartesian coordinates are shown in some of the Figures for the sake ofreference and are not intended to be limiting as to direction ororientation.

The acronym “SWIR” stands for “short-wavelength infrared” and representsan example wavelength range from about 900 nm to about 1700 nm.Likewise, the acronym “MWIR” stands for “mid-wavelength infrared” andrepresents an example wavelength range from about 1700 nm to about 5000nm. The acronym “IR” stands for “infrared” and can include NIR, SWIR andMWIR wavelengths unless otherwise noted. The acronym “RMS” stands for“root-mean square.”

Also in the discussion below, the term “operating wavelength” is denotedA and means a wavelength for which the beam expander and the mirrorstherein are designed to be used. In the examples below, the beamexpander is designed to work for at least three operating wavelengths λin the ultraviolet (UV), visible (VIS) and infrared (IR) ranges,respectively. In an example, the operating wavelengths are those thatcan be generated by a high-power Nd:YAG laser, including by frequencymultiplication and/or frequency modification techniques known in theart.

It will be understood by those skilled in the art that the operatingwavelength has an attendant “operating waveband” or “operatingbandwidth” or “linewidth” AA about the operating wavelength, which in anexample is defined by the bandwidth of the light source (or lightsources) that generates/generate light of the operating wavelengths λ. Atypical linewidth Δλ of an Nd:YAG laser is less than 1 nm.

The alternating layers of HfO₂ and SiO₂ are described herein using theshorthand notation “HfO₂/SiO₂” and in like manner the respectivethicknesses τ_(H) and τ_(S) of the alternating layers are denotedτ_(H)/τ_(S).

The term “high efficiency” as used in connection with the beam expanderdisclosed herein means that the beam expander has an opticaltransmittance T_(BE)>95% at each of the operating wavelengths for whichthe beam expander was designed.

Also, the term “optimally reflect,” when used in connection with a givenoperating wavelength and a section SN of the HfO₂/SiO₂ layers, isunderstood as being measured relative to the other operatingwavelengths, so that the given operating wavelength is understood ashaving a higher reflectance than the other operating wavelengths.

High-Efficiency Beam Expander

FIG. 2 is a schematic diagram of an example beam-expander optical system(“beam expander”) 10 that includes two off-axis mirrors 20 and 30. Themirror 20 includes a mirror substrate 21 with a convex substrate surface22 that supports a reflective multilayer coating 24 having a top surface26. The mirror 30 includes a mirror substrate 31 with a concavesubstrate surface 32 that supports a reflective multilayer coating 34having a top surface 36. In an example, top surfaces 26 and 36 interfacewith an ambient environment 60 that includes air or vacuum having anominal refractive index of n=1. In an example, top surfaces 26 and 36define respective “mirror surfaces” or “reflective surfaces” for mirrors20 and 30.

In an example, reflective multilayer coatings 24 and 34 have the samestructure, in which case the different reference numbers for thecoatings denote on which mirror substrate 21 or 31 the reflectivemultilayer coating resides. As discussed in greater detail below,reflective multilayer coatings 24 and 34 are made up of layers of thedielectric materials HfO₂ and SiO₂. The HfO₂/SiO₂ layers are configuredto enhance the reflectance of mirrors 20 and 30 so that beam expander 10can have a high-efficiency optical transmittance T_(BE) of greater than95% for UV, VIS and IR light.

In an example, beam expander 10 includes a housing H that operablysupports mirrors 20 and 30. In an example, housing H is made ofdiamond-turned lightweight metal, such an aluminum alloy, and mirrors 20and 30 are formed such that they are integral with the housing toprovide mechanical and thermal stability.

In an example, mirror substrates 21 and 31 are made of metal. Examplemetals include non-ferrous metals that can be diamond turned, and inparticular include nickel and nickel alloys, magnesium and magnesiumalloys, copper and copper alloys, and aluminum and aluminum alloys. Inan example, the metal is a lightweight metal such as aluminum, analuminum alloy, magnesium or a magnesium alloy, so that the beamexpander 10 can be made lightweight.

The beam expander 10 is configured to receive a collimated laser beam 40of a first diameter D1 and form therefrom an expanded, collimated laserbeam of diameter D2. The beam expander 10 thus has an afocalconfiguration with a magnification M_(BE)=D2/D1. For beam expansion,M_(BE)>1, i.e., beam expander 10 has greater than unity magnification.In an example, collimated laser beam 40 originates from a high-powerlaser 50 that can emit wavelengths over multiple operating wavelengths(and thus wavelength bands) in the UV, VIS and IR ranges, e.g., 355 nm,532 nm and 1064 nm.

In an example, mirror substrates 21 and 31 are made of a lightweightmetal, such as an aluminum alloy, e.g., Al 6061-T6. In an example,substrate surfaces 22 and 32 are diamond turned to define the respectiveconvex and concave curvatures. As noted above, in an example, mirrorsubstrates 21 and 31 can be defined by housing H and are formedintegrally therewith. In an example, mirrors 20 and 30 are sphericalmirrors, while in other examples they can have different shapes, such asaspheric, cylindrical, anamorphic, etc.

HfO₂/SiO₂ Multilayer Coatings

In an example, reflective multilayer coatings 24 and 34 are each formedfrom alternating layers of HfO₂ and SiO₂, denoted as HfO₂/SiO₂. FIG. 3is a close-up, cross-sectional view of either mirror 20 or mirror 30 andshows a portion of mirror substrate 21 or 31 and the correspondingreflective multilayer coating 24 or 34. The +z-direction isperpendicular to substrate surface 22 or 32 as shown. The HfO₂/SiO₂layers have respective thicknesses τ_(H) and τ_(S), which in analogousfashion are denoted in shorthand as τ_(H)/τ_(S). In the discussionbelow, the HfO₂/SiO₂ layers that define reflective multilayer coatings24 and 34 are divided up into N sections SN, e.g., sections S1, S2, . .. , SN, with section S1 being immediately adjacent (i.e., closest to)substrate surface 22 or 32. The thicknesses τ_(H)/τ_(S) of the HfO₂/SiO₂layers in a given section SN are denoted SN (τ_(H)/τ_(S)) for ease ofdiscussion. The average thicknesses τ_(H)/τ_(S) of the HfO₂/SiO₂ layersin a given section SN are denoted SN(τ_(H)/τ_(S))_(AVG).

A characteristic of reflective multilayer coatings 24 and 34 is that thethicknesses τ_(H)/τ_(S) of the HfO₂/SiO₂ layers generally change withdistance in the +z direction, i.e., in the direction away from substratesurface 22 or 32 to top surface 26 or 36 of the reflective multilayercoating. In an example, the changes in thicknesses τ_(H)/τ_(S) can occurin a stepped fashion, i.e., wherein within each section SN the layerthicknesses τ_(H)/τ_(S) are substantially constant but change fromsection to section. Also, there can be some variation in the layerthicknesses τ_(H)/τ_(S) in a given section SN wherein most but not allof the layer thicknesses τ_(H) are substantially the same and most butnot all of the layer thickness τ_(S) are substantially the same. In anexample, SN(τ_(H)/τ_(S))_(AVG)>SN+1(τ_(H)/τ_(S))_(AVG), i.e., theaverage thicknesses τ_(H)/τ_(S) within a given section SN are greaterthan those of the overlying section SN+1. So, for example, if reflectivemultilayer coatings 24 and 34 are divided into three sections S1, S2 andS3, then in the example,S1(τ_(H)/τ_(S))_(AVG)>S2(τ_(H)/τ_(S))_(AVG)>S3(τ_(H)/τ_(S))_(AVG).

FIG. 4 is a plot of the refractive index n (at λ=355 nm) versus distancez (nm) that shows an example of the varying thicknesses of the differentSiO₂ and HfO₂ layers, as well as the overall thickness of reflectivemultilayer coating 24 or 34. The SiO₂ layers have a refractive index nof about 1.5 while the HfO₂ layers have a refractive index of about 2.1.In an example, the SiO₂ layers generally decrease in thickness τ_(S) instepwise fashion between sections S1 and S3, e.g., from 90 nm≤τ_(S)≤190nm in section S1 to 60 nm≤τ_(S)≤70 nm in section S3. Likewise, the HfO₂layers generally decrease in thickness τ_(H) between sections S1 and S3,e.g., from 130τ_(H) 140 nm in section S1 to 40 nm τ_(H) 50 nm in sectionS3.

Table 1 below summarizes example ranges for the thicknesses τ_(S) andτ_(H) in each of the sections S1, S2 and S3.

TABLE 1 Summary of τ_(S) and τ_(H) for sections S1, S2 and S3 ThicknessS1 (IR) S2 (VIS) S3 (UV) τ_(S)  90 nm to 140 nm 70 nm to 90 nm 60 nm to70 nm τ_(H) 130 nm to 140 nm 50 nm to 70 nm 40 nm to 50 nm

The reflective multilayer coating 24 or 34 of FIG. 4 has an overallthickness TH as measured from substrate surface 22 or 32 to top surface26 or 36 of about 7250 nm. In an example, a relatively thick SiO₂ layer68 may be added as an outermost capping layer that defines top surface26 or 36 and that further increase durability to laser irradiation.Likewise, FIG. 4 shows that the SiO₂ layer immediately adjacentsubstrate surface 22 or 32 is thinner (e.g., 90 nm) than the next SiO₂layer (e.g., 180 nm or 190 nm). So, as emphasized above, there can besome variation in the thicknesses τ_(H)/τ_(S) of the HfO₂/SiO₂ layers ina given section SN.

FIG. 5 is a plot of the reflectance R (%) versus wavelength λ (nm) foran example “triple band” reflective multilayer coating 24 or 34 designedfor use at operating wavelengths λ of 355 nm (UV), 532 nm (VIS) and 1064nm (IR). The horizontal dashed line shows the R=95% reflectance value,and it can be seen that the reflectance R is greater than 95% for eachof the wavelengths λ of interest.

FIG. 6 is a schematic diagram of an example configuration of reflectivemultilayer coating 24 or 34 that illustrates the operation of themultilayer coating in providing such high reflectance at the threeexample operating wavelengths of interest (λ=355 nm, 532 nm and 1064nm), as well as MWIR wavelengths that pass through the reflectivemultilayer coating and reflect from substrate surface 22 or 32. Thereflectance of the MWIR wavelengths from substrate surface 22 or 32(e.g., R>92% or R>95%) enables these wavelengths to be transmittedthrough beam expander 10, e.g., in the opposite direction of laser beam40. In an example, this allows for detecting targets (not shown) at theMWIR wavelengths through beam expander 10, with the signal-to-noiseratio being defined by the beam expansion ratio or magnificationM=D2/D1.

The example reflective multilayer coating 24 or 34 of mirror 20 or 30shown in FIG. 6 is divided into three sections S1, S2 and S3, withsection S1 being configured to provide a high reflectance R (i.e.,optimally reflect) for the NIR operating wavelength of λ=1064 nm whilebeing substantially transmissive to MWIR wavelengths. The close-up insetshows the HfO₂/SiO₂ layers in section S1. The sections S2 and S3 alsoinclude the HfO₂/SiO₂ layers, whereinS1(τ_(H)/τ_(S))_(AVG)>S2(τ_(H)/τ_(S))_(AVG)>S3(τ_(H)/τ_(S))_(AVG).

Section S1 has an HfO₂/SiO₂ multilayer structure with the thickestHfO₂/SiO₂ layers (on average), e.g., the thickness τ_(H) is in the rangefrom 130 nm to 140 nm for most if not all of the HfO₂ layers and thethickness τ_(S) is in the range from 90 nm to 190 nm for most if not allof the SiO₂ layers.

Section S2 is the middle section and is configured to provide a highreflectance R (i.e., to be optimally reflective) in the visible (VIS)operating wavelength of λ=532 nm while being substantially transmissivefor the NIR and the MWIR wavelengths. The HfO₂/SiO₂ layers in section S2have intermediate thicknesses τ_(H) and τ_(S), e.g., a thickness τ_(H)in the range from 50 nm to 70 nm for most if not all of the HfO₂ layersand a thickness τ_(S) in the range from 70 nm to 90 nm for most if notall of the SiO₂ layers.

Section S3 is the uppermost section and is configured to provide a highreflectance R (i.e., to be optimally reflective) at the UV operatingwavelength of λ=355 nm while being substantially transmissive for theVIS, the NIR and the MWIR wavelengths. Section S3 has the thinnestHfO₂/SiO₂ layers, e.g., a thickness τ_(H) in the range from 40 nm to 50nm for most if not all of the HfO₂ layers and a thickness τ_(S) in therange from 60 nm to 70 nm for most if not all of the SiO₂ layers. Tofacilitate manufacturability of the mirror, the UV and VIS bands areconnected in the design. As noted above, a thick SiO₂ layer 68 may beadded as an outmost capping layer atop section S3 to further increasedurability to laser irradiation.

The above-described method of forming reflective multilayer coating 24or 34 in sections SN, with each section configured to have a selectreflectance R for a given operating wavelength, can be used to designmirrors 20 and 30 for use in beam expander 10. FIG. 7 is a plot of thereflectance R (%) versus wavelength λ for an example reflectivemultilayer coating 24 or 34 with a high reflectance at operatingwavelengths λ of 355 nm, 532 nm, and 1064 nm as well as at SWIRwavelengths. FIG. 8 is similar to FIG. 7 and plots the reflectance R (%)at operating wavelengths λ of 355 nm, 532 nm, 1064 nm and at SWIRwavelengths while the substrate surface 22 or 32 has high-reflectance atMWIR wavelengths that are substantially transmitted by the differentsections SN of the reflective multilayer coating 24 or 34.

Controlling Loss Due to Scattering

In an example embodiment, reflective multilayer coatings 24 and 34 eachhas a reflectance of 98% or greater at each of the UV, VIS and IRoperating wavelengths λ, and beam expander 10 has an opticaltransmittance T_(BE)>95%, which makes it a high-efficiency opticalsystem. To achieve this high reflectance for mirrors 20 and 30 and highefficiency for beam expander 10, the amount of loss due to scatteringfor each mirror needs to be controlled.

In the reflectance plot of FIG. 5, a reflectance R of greater than 99.9%can in theory be achieved at all of the “triple bands,” i.e., at λ=355nm, 532 nm and 1064 nm, by having a perfect substrate surface and bycoating interfaces having zero RMS surface roughness.

In practice, however, substrate surfaces and coating interfaces havesome degree of surface roughness that diminishes the reflectance. FIG. 9is a plot of the scatter loss SL (%) for a single mirror 20 or 30 as afunction of the RMS substrate surface roughness MSR_(RMS) (nm) at theoperating (design) wavelengths λ=355 nm, 532 nm and 1064 nm. The plotshows that the scatter loss SL is zero when the RMS substrate surfaceroughness MSR_(RMS) is zero, i.e., for a perfect surface. The scatterloss SL increases as the RMS substrate surface roughness MSR_(RMS)increases. For example, when MSR_(RMS)=3 nm, the scatter loss SL is0.22% at 1064 nm, 0.50% at 532 nm and 1.13% at 355 nm. For MSR_(RMS)=6nm, the scatter loss SL is 0.62% at 1064 nm, 2% at 532 nm and 4.5% at355 nm.

FIG. 10 is a plot of the transmittance T_(BE)(%) of beam expander 10versus the RMS substrate surface roughness MSR_(RMS)(nm). The plot showsthat an amount of RMS substrate surface roughness MSR_(RMS)=3 nm resultsin an optical transmittance T_(BE) for beam expander 10 of T_(BE)=99.75%at λ=1064 nm, T_(BE)=99.00% at λ=532 nm and T_(BE)=97.76% at λ=355 nm.An RMS substrate surface roughness MSR_(RMS)=6 nm results in an opticaltransmittance T_(BE) for beam expander 10 of T_(BE)=97% at λ=1064 nm,T_(BE)=96% at λ=532 nm and T_(BE) 90% at λ=355 nm (obtained byextrapolating the curve for 355 nm to an MSR_(RMS) of 6 nm).

The optical transmittance T_(BE) of beam expander 10 is limited by thescattering loss at the UV operating wavelength of λ=355 nm. In otherwords, according to the plot of FIG. 10, mirrors 20 and 30 of beamexpander 10 each need to have an RMS substrate surface roughnessMSR_(RMS)<4.5 nm for the beam expander to have a high-efficiency opticaltransmittance of T_(BE)>95%.

As discussed above, reflective multilayer coating 24 or 34 has multipleHfO₂/SiO₂ stacks or sections SN, such as sections S1 through S3, whereinthe HfO₂/SIO₂ layer thicknesses τ_(H)/τ_(S) generally decrease sectionby section, from the first or lowermost section S1 closest to mirrorsubstrate 21 or 31 to the top or uppermost section S3 that defines topsurface 26 or 36 of the reflective multilayer coating.

The HfO₂/SiO₂ layers of section S3 are configured to provide highreflectance in the UV at λ=355 nm while being substantially transmissiveto the VIS, NIR and MWIR wavelengths. Forming section S3 as the top oruppermost section ensures that there is the lowest amount of scatteringloss for the shortest operating wavelength. In other words, the UVoperating wavelength has the shortest optical-path length withinreflective multilayer coating 24 or 34, thereby providing the lowestamount of scatter loss. Although this approach may increase the amountof scatter loss at the IR operating wavelength due to the IR lighthaving to traverse the longest optical-path length within reflectivemultilayer coating 24 or 34, the scatter loss is not as sensitive atthis longer NIR wavelength as compared to at the shorter UV wavelength.

Thus, the configuration of reflective multilayer coating 24 or 34 suchas is shown in FIG. 6, where the top or uppermost section S1 defines thereflectance for the shorter UV operating wavelength and the bottom orlowermost section S3 defines the reflectance of the longer IR operatingwavelength, is exploited to optimize the optical transmittance T_(BE) ofbeam expander 10 and achieve the high-efficiency optical transmittanceof T_(BE)>95%.

Forming the HfO₂/SiO₂ Layers

The plots of FIGS. 9 and 10 indicate that achieving an opticaltransmittance T_(BE)>95% for beam expander 10 requires that the amountof RMS substrate surface roughness MSR_(RMS) be maintained below acertain threshold value TV, such as TV=4.5 nm in the example discussedabove. This requires forming the HfO₂/SiO₂ layers in each of thereflective multilayer coatings 24 and 34 so that they are as smooth aspossible.

In an example, the HfO₂/SiO₂ layers are formed using the systems andmethods disclosed in the '450 patent. In particular, in an example, theHfO₂/SiO₂ layers are formed on the respective diamond-turned andoptically polished substrate surfaces 22 and 32 of mirror substrates 21and 31 using plasma ion assisted deposition (PIAD) in combination withrotating the respective mirror substrate and employing a “reversed mask”process. This method ensures that reflective multilayer coatings 24 and34 do not increase the RMS substrate surface roughness MSR_(RMS) beyondthat of the original diamond-turned and optically polished substratesurfaces 22 and 32.

Thus, in an example, the diamond-turned and optically polished substratesurfaces 22 and 32 of mirror substrates 21 and 31 each has a RMSsubstrate surface roughness MSR_(RMS) less than a threshold value TV,such as the aforementioned 4.5 nm, which results in an opticaltransmittance T_(BE)>95% for all the operating (design) wavelengths λ.Such threshold values TV are readily achievable using diamond-turningand polishing of an Al surface.

Mitigating Surface Defects

In some instances, substrate surface 22 or 32 can have surfaceimperfections or defects that need to be smoothed out or otherwisemitigated to achieve a high-efficiency optical transmittance T_(BE) forbeam expander 10. For example, metal inclusions are sometimes formed inaluminum (Al) alloys to increase the mechanical strength. The metalinclusions can have a hardness different from that of the bulk Al alloyof mirror substrate 21 or 31 and can appear in the form of small (e.g.,submicron-size) particles on the polished substrate surface 22 or 32.Such surface defects can increase scatter loss SL and may also reducethe laser-induced damage threshold, especially at the UV operatingwavelength.

In one example, surface defects can be mitigated by the depositing of anSiO₂ layer on substrate surface 22 or 32 to seal the surface defectwhile eliminating defect lateral growth with an inversed mask, followedby the smoothing of the SiO₂-coated surface via plasma-ion etching. Inanother example, a pure Al film is deposited on substrate surface 22 or32 to seal the surface imperfection and homogenize the surface, followedby optical polishing the pure Al film. In another example, a layer ofaluminum is deposited on substrate surface 22 or 32 to seal the surfaceimperfection and homogenize the surface, and then the Al-coated surfaceis smoothed via plasma-ion etching. In another example, a metal such asnickel or a nickel alloy is deposited on substrate surface 22 or 32 andthen the coated substrate is processed to define the require curvatureto within a desired surface roughness.

FIGS. 11A through 11C are schematic side views of a portion of anexample mirror 20 or 30 that illustrates an example method of performingsurface-defect mitigation when the mirrors are formed. In FIG. 11A,substrate surface 22 or 32 of mirror substrate 21 or 31 includes surfacedefects 64. In FIG. 11B, substrate surface 22 or 32, and the surfacedefects 64 thereon, are coated with an SiO₂ layer 70 using a reversedmask process. The SiO₂ layer 70 is then plasma-ion etched using aplasma-ion etch process that employs a plasma 80 to form a flat (i.e.,substantially defect-free) surface 72. The reflective multilayer coating24 or 34 consisting of the aforementioned HfO₂/SiO₂ layers is thenformed atop surface 72 of processed SiO₂ layer 70, as shown in FIG. 11C.The optional SiO₂ layer 68 is also shown in FIG. 11C as being depositedatop the uppermost section S3 of reflective multilayer coating 24 or 34.

The resulting mirrors 20 and 30 are then employed in beam expander 10 toachieve the multiwavelength performance over the UV, VIS and NIRwavelengths with a high-efficiency optical transmittance T_(BE)>95%.

An advantage of beam expander 10 disclosed herein is that reflectivemultilayer coatings 24 and 34 can have a relatively high laser-induceddamage threshold at all the operating wavelengths λ. The HfO₂/SiO₂layers are formed to be dense and smooth to reduce or otherwise minimizescatter loss SL while also being resistant to laser damage andenvironmental erosion. When necessary, substrate surface defectmitigation is carried out as described above to reduce or eliminate theadverse effects of substrate surface defects on the opticaltransmittance T_(BE) of beam expander 10. Further, because reflectivemultilayer coatings 24 and 34 are formed in sections SN that aredesigned to reflect a particular operating wavelength λ whilesubstantially transmitting other wavelengths, substrate surfaces 22 and32 of mirrors 20 and 30 can have a relatively high reflectance at MWIRwavelengths.

It will be apparent to those skilled in the art that variousmodifications to the preferred embodiments of the disclosure asdescribed herein can be made without departing from the spirit or scopeof the disclosure as defined in the appended claims. Thus, thedisclosure covers the modifications and variations provided they comewithin the scope of the appended claims and the equivalents thereto.

What is claimed is:
 1. A method of forming a high-efficiencybeam-expander optical system for use at ultraviolet (UV), visible (VIS)and infrared (IR) operating wavelengths, comprising: diamond-turning andpolishing a first mirror substrate and a second mirror substrate torespectively form a first mirror having a convex substrate surface and asecond mirror having a concave substrate surface; forming on each of theconvex substrate surface and the concave substrate surface a reflectivemultilayer coating comprising alternating layers of HfO₂ and SiO₂ havingrespective layer thicknesses τ_(H) and Is, including arranging the HfO₂and SiO₂ layers in at least three sections S1, S2 and S3 in orderoutward from the convex substrate surface or the concave substratesurface, with the three sections S1, S2 and S3 respectively configuredto optimally reflect the IR, VIS and UV operating wavelengths; andarranging the first mirror and the second mirror in an off-axis, afocalconfiguration having greater than unity magnification and an opticaltransmittance T_(BE)>95% at each of the UV, VIS and IR operatingwavelengths.
 2. The method according to claim 1, the forming thereflective multilayer coating comprising alternating layers of HfO₂ andSiO₂ comprises using plasma ion assisted deposition (PIAD).
 3. Themethod according to claim 2, wherein the plasma ion assisted deposition(PIAD) comprises rotating the first mirror substrate and the secondmirror substrate.
 4. The method according to claim 2, wherein the plasmaion assisted deposition (PIAD) comprises employing a reversed maskprocess.
 5. The method according to claim 1, wherein at least one of theconvex substrate surface and the concave substrate surface includes oneor more surface defects, and further including: coating the at least oneof the convex substrate surface and the concave substrate surface withan SiO₂ layer; plasma processing the SiO₂ layer; and forming thereflective multilayer coating comprising alternating layers of HfO₂ andSiO₂ atop the plasma-processed SiO₂ layer.
 6. The method according toclaim 1, wherein the diamond-turning and polishing is carried out sothat the convex substrate surface and the concave substrate surface eachhas a root-mean-square (RMS) substrate surface roughness MSR_(RMS) ofless than 4.5 nm.
 7. The method according to claim 1, wherein the firstmirror substrate and the second mirror substrate are each formed from analuminum alloy, and wherein the layer thicknesses τ_(H) and τ_(S) insection S1 are on average greater than those in section S2 and the layerthicknesses τ_(H) and τ_(S) in section S2 are on average greater thanthose in section S3.
 8. The method according to claim 1, wherein sectionS1 is substantially transmissive to a mid-wavelength IR (MWIR)wavelength, section S2 is substantially transmissive to the IR operatingwavelength and the MWIR wavelength, and section S3 is substantiallytransmissive to the VIS and IR operating wavelengths and the MWIRwavelength.